It sounds like this is a project that should be left to the professionals. You do not overfill an air conditioner system because doing so can destroy the compressor. You do not use a vacuum pump to remove some of the refrigerant. That is not its purpose. Also, gauges aren't going to tell you anything useful.
First you have to understand that Ford uses a lot of push-together pipe connectors with a rubber o-ring and spring inside. Those almost always leak. It is just a matter of how fast they let the refrigerant leak out.
An important thing to be aware of is the refrigerant is always part liquid and part vapor. The liquid fills about half of the evaporator in the dash. The top of that level is where it evaporates, (hence the term "evaporator"), and that process requires heat to be absorbed. To say it a different way, that is where it becomes very cold. If you over-charge the system, the level of the liquid will be higher and it will turn to a vapor further up in the evaporator. It doesn't take much for the point of evaporation to occur in the hose going out of the firewall. The result is the point where cooling takes place is under the hood. Inside you'll get uncooled air. Worse yet, air conditioner compressors cannot handle a liquid. They will be damaged if liquid tries to go through the valves. That liquid can slosh into the compressor if there is too much in the system.
Besides leaking connections, Ford has another problem. For decades Chrysler used a sight glass on the receiver / drier to give a quick and simple indication if the system was low on charge. Vapor bubbles would be seen flowing through the glass. Ford finally started using a sight glass too around the late 1980's or early 1990's. The problem is when their systems are fully-charged, there will still be vapor bubbles in the sight glass, so they really have no value. If you try to get rid of the bubbles, you will keep charging and charging, but you will never get rid of them.
The next issue has to do with the refrigerant itself. The older R-12 had an interesting characteristic that in a sealed container or a sealed air conditioner system, when it was at rest, the pressure was almost exactly the same as the temperature. That means if the car was sitting in a garage out of the sun, and it was 70 degrees F, the pressure in the system would be 70 pounds. The R-12 was not designed to make that happen. It just was the nature of the beast. I don't know how the newer R-134 correlates to that, but it looks like it has a very similar characteristic.
The important point to understand related to that pressure is if you do have 70 pounds in the system, for example, and you bleed some off, the pressure will go down, some of the liquid will turn to vapor and expand, and the pressure will go right back up to 70 pounds. This will keep on happening no matter how much you bleed off, as long as there is still some liquid in the system. That means you can have one percent of a full charge and have 70 pounds, or you can have 150 percent of a full charge, and you'll still have 70 pounds.
I know this can be hard to believe at first, but it can be compared to your cooling system too. Why are we warned to not remove the radiator cap while the engine is hot? It's not (only) because you will burn your hand. Related to that, how can coolant get to 235 degrees and not be boiling? How can NASCAR engines use straight water and reach 260 degrees? It is the same reason Mom used to use a pressure cooker to cook some food. Water boils at 212 degrees, but that is at sea level where the weight of the air in the atmosphere puts it under pressure. For every pound more of pressure, the boiling point goes up three degrees. That means a 15 pound radiator cap increases the boiling point of the water by 45 degrees, or to a total of 257 degrees. If your engine gets up to 220 degrees, which is very common, and you remove the radiator cap, that 15 pounds of pressure is gone, and the boiling point instantly drops to 212 degrees. 220 degree water will instantly turn to steam, (vapor), and expand hundreds of times. That severe expansion is what blows the hot antifreeze all over you.
If you were able to quickly put the cap back on, the water that turned to steam and expanded would have expanded so much that the pressure in the system would go right back up, and the boiling would stop.
Now lets get back to refrigerant. It also has a boiling point, but that is way below 0 degrees. Again, we will assume it is at 70 pounds of pressure. You bleed some vapor off, just like removing the radiator cap, and the pressure goes down. That lowers the boiling point of the refrigerant, but that is irrelevant in this sad story because it's going to never get below its boiling point where we live. The point is, the pressure will go down momentarily, and that will cause more liquid to turn to vapor and expand, and the pressure will go right back up. You can bleed off one pound, two pounds, or 99 percent of the full charge, and the pressure will still be 70 pounds in the system. Once 100 percent of the refrigerant has turned to a vapor, you can start to look at it like air in a tire. As you bleed some off, the pressure will go down.
All of this story refers to a refrigerant system that is at rest. What mechanics are watching on the gauges is what happens when the system is not at rest. There is no way to tell with the gauges if the system is fully-charged. What we're looking for is if the "high side" pressure goes up immediately when the compressor kicks in, and how fast it drops when the compressor turns off. We can get sort of an indication of state of charge by watching the low side pressure, but that can be misleading. The low side should drop when the compressor is sucking it and pumping it higher on the other side, but there will always be something watching to insure that low-side pressure never gets too low. On many cars it is that low side pressure that is used to determine when to cycle the compressor on and off. The compressor will be turned off when the low side gets to around 40 degrees. Remember that its temperature very closely follows the pressure. 40 degrees would be very uncomfortable in a car, but we are blowing real warm air through that evaporator. The issue here is the humidity in the air will condense on the cold evaporator. That is the water you see dripping onto the ground by the right front tire. If the low side pressure were allowed to drop to 32 pounds, the temperature of the evaporator would get close to 32 degrees, and that condensed water would freeze into a block of ice and block air flow. Some cars use a temperature-sensing bulb filled with an expandable gas to control the expansion valve, and thereby control refrigerant flow and evaporator temperature. Today a lot of cars use electronic controls. Regardless of the method used, the goal is always the same, ... Prevent the evaporator from getting too cold.
The vacuum pump you mentioned is never used to remove refrigerant. When an air conditioner system has to be opened to be worked on, the refrigerant is drawn out first with a recovery machine. It can be recycled and reused later. While the system is open, air gets in along with the humidity in it. That water vapor is a killer to air conditioner systems. If my training and memory serve me right, R-12 and water combine to form hydrochloric acid, and that is extremely corrosive to the metal evaporator, condenser, and metal pipes.
A second problem is if there were to be some water in the system, the receiver / drier is capable of collecting up to about ten droplets of that water and holding it in suspension so it does not travel around the system. Where the refrigerant goes through the expansion valve or orifice tube, it gets real cold, but it is still a liquid. A drop of water would freeze at that point and stop the flow. The common symptom is nice cold air for a little while, then only warm air until that ice melts and flow is restored. You might get ten or twenty-minute bursts of cool air in between twenty minutes spells of warm air.
To prevent these problems, once repairs are completed, we attach the vacuum pump to put the system into a vacuum so water vapor is easier to draw out. At a near-perfect vacuum, water will boil at 77 degrees F. That is the correlation to raising the boiling point when water is under pressure. Here is where the gauges are valuable. We use the vacuum pump to maintain vacuum for about a half hour. When the pump is turned off, we want to see that vacuum hold for a long time. Since time is money to our customers, we only watch for a minute or two, but that is enough time to see if the vacuum is being lost. Two things can cause that. First, if there's still water in the system, and it's below 77 degrees in the shop, it may still be boiling and turning to a liquid. If it is, it is still expanding. That will cause a reduction in vacuum. The solution for that is simply to run the vacuum pump a while longer, and perhaps run the engine to warm up the hoses and other components.
The second cause of losing vacuum is if there is a leak. To find that we have to inject a partial refrigerant charge, (remember, we only need enough that some remains a liquid), then search with an electronic leak detector, soap bubbles, or with dye in the system, and a black light. Once all the problems are solved, the final step is again to pump the system into a vacuum, and when it holds, fill it with the measured amount called for.
That brings me to my final point of great value. The only way to know for sure if the system has the right amount of refrigerant is to inject a measured amount into a system that started with 0.0 pounds. The vacuum we started with will cause as much as a pound to be drawn in. After that we have to rely on the pump in the charging station or the fact the refrigerant in the tank is at a higher pressure than what is in the air conditioner system low side when the compressor is running. If you are using the little one-pound cans, you won't get the entire contents to go into the system. A quick way to overcome most of that is to set the can in a bowl of hot water. That will raise the temperature and therefore the pressure by quite a bit. You can get almost everything out of each can that way.
Remember that "more is better" only refers to horsepower! More refrigerant will not get colder and it will not produce more volume of cooled air. As the volume and temperature of the incoming air changes, the flow of the refrigerant will be automatically adjusted to keep pace with demand for cooling. Too little refrigerant will cause the liquid to turn to vapor before it gets into the evaporator. Too much refrigerant will cause the liquid to turn to vapor after it has left the evaporator. Either condition will result in the point of cooling being under the hood. The engine might appreciate that on a hot day, but that really isn't the point of having air conditioning. The correct charge will put the point of evaporation right in the middle of the evaporator where the air will be blowing through.
I must mention too that refrigerant is extremely dangerous to work with. It can cause blindness and frostbite. Professionals always wear safety glasses and gloves, and the smart ones also wear a face shield.
Wednesday, June 22nd, 2016 AT 10:34 PM